树形便利。
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
void solve(TreeNode *root, int &ans, int k, int &index)
{
if (root->left != nullptr && index <= k)
solve(root->left, ans, k, index);
if (index == k)
ans = root->val;
index++;
if (root->right != nullptr && index <= k)
solve(root->right, ans, k, index);
}
int kthSmallest(TreeNode* root, int k) {
int ans = 0;
int index = 1;
solve(root, ans, k, index);
return ans;
}
};